This invention relates to an air blast fuel nozzle system for gas turbine engines, and is particularly adapted for use with recuperated gas turbine engines.
Gas turbine engines in general are widely known in the art, and commonly comprise a compressor and expansion turbine coupled together for simultaneous rotation on a common shaft. Alternately, some gas turbine engines include multiple compressor sections and/or multiple turbine sections which may include a free power turbine. In any event, the compressor functions to compress ambient air for supply to a combustor for atomization with fuel and subsequent ignition. The combustion of the fuel-air mixture results in high energy exhaust gases which are expanded through the turbine for obtaining power output, and for rotatably driving the compressor.
For improved efficiency, it is well known to employ a recuperating heat exchanger in combination with the basic gas turbine components. That is, a heat exchanger, or recuperator, is interposed between the exhaust side of the expansion turbine and ambient to pick up and retain a substantial portion of the heat energy remaining in the exhaust gases. The heat energy is transferred within the recuperator to the compressed air from the compressor in order to substantially elevate the temperature thereof. The compressed air ultimately supplied to the combustor via the recuperator thus is at a substantially higher energy level than it otherwise would be were the recuperator not employed. In this manner, substantially less fuel in the combustor is required to raise the combustor exhaust gases to design temperature limits resulting in substantially improved engine fuel economy.
Prior art recuperated gas turbine engines typically are designed to have a relatively low pressure drop across the combustor for optimized engine performance. Hence, the available pressure drop across a combustor fuel nozzle is also relatively low, since the fuel nozzle pressure drop corresponds with the pressure differential across the combustor. This results in marginal, unsatisfactory fuel atomization during engine starting and certain acceleration modes when fuel flow is relatively high compared to fuel nozzle pressure drop. At these conditions, insufficient air atomization energy results in ignition difficulties and poor engine efficiency, particularly when relatively high viscosity fuels such as diesel fuel are used. These ignition problems are compounded by the requisite air pressure drop across the recuperator, and are further compounded during high altitude and/or cold ambient operating conditions.
Prior art systems have been developed including, for example, nozzle assist devices for providing pressurized "secondary" air to a fuel nozzle during starting conditions to momentarily improve fuel atomization. However, these devices add significantly to the cost and complexity of the fuel nozzle system, and are of little value at other modes of engine operation such as acceleration, etc. Other relatively complicated auxiliary fuel nozzle systems have also been proposed which employ specially designed fuel nozzle systems for adequate atomization during starting, and which may be fed with high pressure gas as from pressurized cartridges or the like in order to start the engine. However, these special starting nozzles normally must be disabled after the engine is started, and conventional "run" nozzles supplied with recuperated discharge air are employed during "running" conditions. Again, the special nozzle designed for improved atomization are of little value at other operational conditions such as acceleration wherein improved atomization is required. See, for example, U.S. Pat. Nos. 2,574,495 and 3,095,705. Accordingly, these systems require duplicate fuel nozzle arrangements which undesirably raise the cost and complexity of the engine, can contribute to fuel nozzle fouling, and yet do not completely satisfy atomization requirements at all conditions of engine operation.
This invention overcomes the problems and disadvantages of the prior art by providing a fuel nozzle system including a selectively operated supercharging boost pump for controllably supplying non-recuperated boosted compressor discharge air to the nozzle when required for improved fuel atomization.